SAN FRANCISCO (AP) — The margin for error is exceedingly small for a sailor sitting three stories above the San Francisco Bay on the side of a high-tech catamaran as it leans toward the water literally flying along the surface like a giant stingray at 40 mph.

It doesn't take much for these top-heavy craft to topple — it's already happened twice in these waters since October.

The days of tanned sailors in Sperry Top-Siders boat shoes, baseball caps and shorts furiously changing sails amid a spaghetti mess of ropes are long gone. They've been replaced by billionaire Larry Ellison's made-for-television vision of fixed-sail yachts equipped with cutting-edge technology controlled by professional athletes who rely more on computers than ropes.

Following a sailor's death two weeks ago after a capsize during an America's Cup training run on San Francisco Bay, even more safety gear is being mandated and gladly donned by sailors, some of whom privately predict more capsizes and crashes before a winner is crowned in September.

New safety regulations proposed last week addressed capsizes at length and eased competitive rules in favor of assisting an overturned yacht. The number of round robins in the first round of sailing among the challengers has been reduced from seven races a series to five.

On Friday, three of the four America's Cup entries took to the water for the second day of practice runs since Andrew "Bart" Simpson's death on May 9. Each of the 72-foot catamarans on the water was followed closely by an armada of chase boats carrying scuba divers, doctors and other support personnel in case one of the space-age vessels capsized. Simpson died when he was trapped under the wreckage of Artemis Racing's capsized boat.

No one was hurt when Oracle Racing's yacht capsized in October, but it required millions of dollars in repairs after its sail was destroyed.

Since Ellison's boat won the last America's Cup in 2010, the Oracle Corp. founder designed the race course and boats that would compete this summer. When he unveiled the plan to race some of the world's fastest sail boats on a tight course between San Francisco's iconic Alcatraz Island and Fisherman's Wharf, he had hoped as many as 12 challengers would sign up to face him. Only three materialized. Those that passed said they were put off by the cost and the complicated specifications of the catamarans powered by airplane-like wings instead of traditional, flapping sails.

Each boat costs upward of $10 million and three of the four teams built two yachts apiece. Each team employs about 100 full-time workers at water-front bases equipped with giant cranes and other heavy equipment to move the boats from their storage berths to the water and back again for every training run.

"It's too expensive," Team New Zealand leader Grant Dalton grumbled before heading out for the team's first sustained practice on the bay in light winds Friday morning. "It's too complex."

Dalton promised to simplify the next America's Cup if his teams' derisively dubbed "sailing billboard" for all of its NASCAR-like advertising beats Ellison's boat in September.

"We don't have a billionaire backer," said Team New Zealand spokesman Hamish Hooper. The team is the only America's Cup entry to receive government backing with lawmakers in sailing-crazed New Zealand chipping in about $32 million, about the third of the cost of supporting a team.

The other three teams are backed almost entirely by a single wealthy funder: Ellison pays for Oracle Racing, Swedish oil magnate Torbjörn Törnqvist backs Artemis Racing and Prada owner Patrizio Bertelli funds the Luna Rossa Challenge.

One of Artemis' two boats capsized and broke into pieces, trapping Simpson under wreckage under water for more than 10 minutes. Artemis chief executive Paul Cayard said his team still plans to compete for the cup, but only if conditions are deemed safe. Artemis has not sailed since the accident while the other three teams returned to practice this week after observing a brief moratorium on sailing after Simpson's death.

On Friday morning, the winds were light and stubbornly refusing to pick up, fluttering between 8 and 12 knots. The recreational sail boats out on the bay not using their motors were bobbing with the current in the sunshine. The America's Cup catamarans, on the other hand, were each reaching speeds of 30 knots as they whizzed up and down San Francisco's waterfront with the flotilla of motor boats opening their throttles wide to keep up.

Though this was only New Zealand's second run on San Francisco Bay, they've completed more than four dozen practice runs at home. They've mastered "foiling," the technique of lifting the 7-ton boat's hulls out of the water so it can skim along the waves on four small fins known as "foils." Foiling reduces the drag on the boat and increasing speed dramatically.

On Friday, New Zealand's black-clad rigger was being hoisted in the air to affix the catamaran's front sail when Ellison's 2003 America's Cup entry came into sight.

Called USA 76, the single-hulled boat with the mainsail and jib setup of a traditional sloop now serves as tour boat and the grinning guests aboard eagerly waived and yelled their hellos to the kiwis.

Someone asked Team New Zealand's David Thomson the difference between the 2003 boats and the entries of today.

"That's a dog," said Thomson, gesturing toward the USA 76. He then turned and pointed at New Zealand's sleek catamaran with the rigger back on deck, the jib sail in place and the boat picking up as it glided toward the Golden Gate Bridge. "And that's a cat."

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AP sportswriter Bernie Wilson contributed to this report from San Diego.

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